It is well known in the art to harden ferrous materials, such as medium carbon steel, by heating the material to a high temperature, below its melting temperature, and subsequently quenching it, that is, cooling it rapidly enough to form hard martensite. Heating can take place in furnaces or by induction heating, and cooling can take place by applying a cooling fluid, such as water or water mixed with other components.
Often, it is only the surface that needs to be hardened. Surface hardening increases the wear resistance of the material and can sometimes also be used to increase fatigue strength caused by residual compressive stresses. Surface hardening can be useful for hardening surfaces that will be subjected to substantial wear when in use, for example, bearing surfaces, such as journal surfaces of crankshafts.
Laser surface hardening is a method of surface treatment in which high energy laser light is employed as a heat source to harden the surface of a substrate. It is known to use laser light to achieve surface hardening, cf., for example:    F. Vollertsen, et al., “State of the art of Laser Hardening and Cladding”, Proceedings of the Third International WLT-Conference on Lasers in Manufacturing 2005 Munich, June 2005;    M. Seifert, et al., “High Power Diode Laser Beam Scanning in Multi-Kilowatt Range”, Proceedings of the 23rd International Congress on Applications of Lasers and Electro-Optics 2004;    S. Safdar, et al., “An Analysis of the Effect of Laser Beam Geometry on Laser Transformation Hardening”, Journal of Manufacturing Science and Engineering, August 2006, Vol. 128, pp. 659-667;    H. Hagino, et al., “Design of a computer-generated hologram for obtaining a uniform hardened profile by laser transformation hardening with a high-power diode laser”, Precision Engineering 34 (2010), pp. 446-452;    U.S. Pat. No. 4,313,771-A;    DE-4123577-A1;    EP-1308525-A2;    EP-2309126-A1;    JP-2008-202438-A;    JP-S61-58950-A;    U.S. Pat. No. 4,797,532-A.
Using laser light for surface hardening involves several advantages: the laser beam is essentially independent of the workpiece, is easily controlled, requires no vacuum, and generates no combustion products. Also, as the laser beam generally only heats the metal product or workpiece locally, the rest of the workpiece can act as a heat sink, assuring rapid cooling, which is also known as self-quenching: the cold interior of the workpiece constitutes a sufficiently large heat sink to quench the hot surface by heat conduction to the interior at a rate high enough to allow martensite to form at the surface. Thus, the need for external cooling media, such as cooling fluids, can be obviated.
One problem involved with the use of laser light as the heat source in metal hardening processes is that the width of the hardening zone is limited by the dimensions of the laser spot. It is known to use optics to modify the shape of the spot, for example, to provide a substantially rectangular spot having a more or less uniform intensity distribution. As an alternative, scanning means (such as a scanning mirror associated with drive means) can be used to repetitively move the spot over the track, so that the heat source can be considered a rectangular source moving along the track.
In spite of its advantages, laser hardening is often not used because it is believed that the production rate will not be high enough for many practical applications of this technique, and because it difficult to achieve that all the parts that are to be heated will be heated to the desired extent. Correct heating is essential to make sure that hardening and tempering is achieved, with the necessary depths, but without causing damage by overheating.
For example, a crankshaft (the part of the engine that translates reciprocating linear piston motion into rotation) is a complex product that has often been conceived as difficult to harden by laser light. An example of a crankshaft is shown in FIG. 1. The crankshaft 1000 is a forged or casted steel product, having two or more centrally-located coaxial cylindrical journals 1001 (also known as the “main journals”) and one or more offset cylindrical crankpin journals 1002 (also known as “rod journals”), separated by counterweights and webs that establish walls 1005 extending substantially perpendicularly to the surfaces of the journals. The complex shape of the product can make it difficult to correctly “scan” the surface with the laser beam; the tracks or areas to harden can have different widths and/or be asymmetric and/or be arranged in different planes (which is the case with the walls 1005 and the surfaces of the journals 1001 and 1002). Thus, today, high-frequency induction heating followed by a polymer-based water quench process is frequently used for the hardening of crankshafts. However, this process, although proven to be useful for achieving the desired hardening, involves certain drawbacks. For example, the inductors for creating heating by induction have to be designed in accordance with the specific design of the crankshaft, which reduces flexibility: to adapt an induction machine to a new kind of crankshaft can be time-consuming and costly. Further, heating by induction is costly in terms of the energy required to heat the crankshaft to the desired extent. Additionally, the cooling process is complex, costly and challenging from an environmental point of view, due to the use of large amounts of cooling fluid that are needed. Besides, parameters such as cooling fluid temperature and flow have to be carefully controlled to ensure a correct hardening process.
Thus, hardening using laser light as the heat source can be an attractive alternative in terms of flexibility, environmental-friendliness, energy consumption, and costs.
DE-10 2005 005 141-B3 discloses a method for laser hardening of the surfaces of the journals of a crankshaft. According to this method, a six-axis industrial robot is used to hold the crankshaft and to subsequently rotate it around the axis of the main journals and around the axes of the rod journals, during heating of the respective journals with laser light. Thus, by using the capacities of movement of the industrial robot, the distance between the laser source and the surface onto which the laser beam is projected can be kept constant.
Also US-2004/0244529-A1 teaches the use of laser to harden a small region of a crankshaft. In this case, laser light is used to harden a plurality of spaced portions, wherein the extent of the portions varies over the region to be hardened. As only a minor portion of the crankshaft is hardened with these spaced portions, there is no need to concern about overheating of other, more heat sensitive portions.
DE-3905551-A1 teaches a system for hardening of a surface of a crankshaft, where a laser beam is projected onto a crankshaft and wherein there is a relative movement between the beam and the crankshaft such that the beam will subsequently be projected onto different portions of the crankshaft. The power or power distribution in the beam is adapted depending on the geometry of the respective portion of the crankshaft and depending on the desired depth of penetration of the laser beam. A problem with the approach taught by DE-3905551-A1 is that it may not allow for a high production rate. To achieve a sufficient depth of the hardened layer (in the motor industry, typically hardening depths of at least 800, 1000, 1500, 2000 or even 3000 μm are required in terms of effective case depth, and it is often desired to have 100% transformed martensite until depths of 200 μm or more), it is not enough to raise the temperature of a certain portion of the surface, but energy has to applied for a sufficiently long time to heat not only the surface, but also the material under the surface, to a sufficient depth. As an excessive heating of the surface is not desired, to achieve the desired penetration the best solution is not to simply increase the amount of power of the laser beam, but rather the time during which the laser heating is applied to the relevant area. In the system disclosed in DE-3905551-A1, where the laser beam is kept stationary and applied to a specific area, obtaining an adequate heating and penetration over the major portions of the main journals or rod journals would appear to require substantial amounts of time. Thus, DE-3905551-A1 may describe a method appropriate for hardening very specific portions of the surface of a crankshaft, but not for hardening the general surfaces of the journals.
Also EP-1972694-A2 focuses on the hardening of specific portions of a crankshaft, namely, of the fillet portions, using one or more lasers. The laser light is directed onto the portion to be hardened and the crankshaft is rotated. The disclosed method can include a pre-heating step, a main heating step, and a post-heating step. It appears that the laser irradiation is maintained constant while rotation of the crankshaft takes place. EP-1972694-A2 is silent on the risk of overheating of more heat sensitive portions of the surface of the crankshaft.
US-2004/0108306-A1 acknowledges that automakers use the induction heating process to harden the bearings of a crankshaft, that is, the surfaces of the main journals and the rod journals, while a mechanical rolling process is utilized to roll the fillets to improve compressive stresses. However, according to US-2004/0108306-A1, these processes are said to be capital-intensive, time-intensive, lead to nonuniformities, and have a crack propensity in the oil lubrication holes that require a tempering process. US-2004/0108306-A1 teaches a fillet heat treatment by laser which aims at eliminating the need for the mechanical rolling process. Closed-loop temperature control by using an optical pyrometer is proposed. The use of a controllable x,y mechanism for maintaining a fixed heating distance between laser and fillet is proposed.
S. M. Shariff, et al., “Laser Surface Hardening of a Crankshaft”, SAE 2009-28-0053 (SAE International), discusses the laser surface hardening of a crankshaft aiming at a hardened case-depth of above 200 μm with a hardness of 500-600 HV at different locations mentioned. The document mentions the problem of melting at the periphery of holes due to reduced heat-sink effect and accumulation of heat at the edge. It is stated that the problem can be dealt with by reducing the pre-heating effect at the hole-edge by choosing an appropriate start-up location and varying process parameters within the permissible range.
One reason for which laser hardening has not become more frequently used in the context of complex products such as crankshafts is that it is believed that it can be difficult to achieve a correct heating of the parts, that is, a sufficient heating to assure correct hardening (generally the hardened layer has to have an effective case depth of at least 800 μm or more, such as at least 1000, 1500, 2000 μm or more, and/or featuring 100% transformed martensite until a depth such as 200 μm or more) while avoiding overheating of sensitive portions. For example, in the case of a crankshaft such as the one of FIG. 1, care must be taken in what regards the heating of the journals in correspondence with the oil lubrication holes 1003 and optionally also in what regards the fillets 1004. For example, if a large laser spot is simply projected onto the surface of the journal during rotation of the journal to heat the entire surface, and if the rotation speed and the power of the laser beam are kept constant so that each portion of the surface receives the same amount of energy, and if this energy is sufficient to achieve an adequate heating of the major part of the surface to produce the desired hardening, the heating may become excessive at the edges of the oil lubrication holes, thus damaging said edges. The same can occur at the fillets, which are commonly undercut; thus, there are edges that can suffer damage if overheated.